The present invention relates to an amperometric or differential current transformer equipped with a system capable of improving its cooling.
Moreover, the present invention relates to a protection device of an electrical circuit, for example, a low voltage one, against an overcurrent, a short circuit or a earth leakage current and that comprises such transformer, to a circuit breaker that comprises such transformer and/or such differential protection device, and to an electrical system comprising such circuit breaker.
As known, circuit breakers or similar devices are devices designed for allowing the correct operation of specific parts of electrical systems and of the installed loads. To this end, they are equipped with suitable protection devices, for example, electronic devices protecting against overcurrents, short circuits or differential currents (earth leakage currents or ground fault currents).
Such protection devices, also indicated simply as “relays” or “trip units,” can be realized and used as stand-alone components, or more typically they are inserted inside the shell of an automatic circuit breaker and are operatively coupled to its breaking part. The relays are normally associated with some current transformers or amperometric transformers (TA) or current transformers (CT). Normally, the current transformers provide the protection unit with a signal indicating the circulating current at each pole of the circuit breaker; in addition to, or as an alternative to this function, the current transformers are used to supply power to the same protection devices.
Similarly, also the protection devices, in particular the differential type, also referred to simply as differentials or differential relays, can be produced and used as stand-alone components, or more typically are associated with the shell of an automatic circuit breaker and are operatively coupled to its breaking part. The most common components of the amperometric transformers, whether of the unipolar or differential type, comprise a toroidal core, or shortly toroid, on which the so-called secondary windings are positioned; the core is then positioned in such a way as to be passed through, depending on the type of use, by one or more electrical conductors which constitute the so-called primary conductors or windings, each of which is directly or indirectly connected to a corresponding phase of the electrical circuit inside which the device is inserted.
One of the critical issues related to the amperometric transformers, in particular those applied to the automatic circuit breakers, is that the electrical junctions in the conductors that pass through the toroid cause local increases in electrical resistance with resulting production of heat. The heat generated is damaging to the life of the transformer and in particular the delicate secondary windings and their insulation coating. The heat also negatively affects the toroidal core, causing undesirable alterations of the typical B-H response curves. Also, when the device is inserted inside a circuit breaker, this undesirable heat contributes to increase the temperature of the circuit breaker and then can negatively affect its operation and performance. It also needs to be noted that when the amperometric transformers are connected directly to the output terminals of the circuit breaker, because of thermal conduction phenomena, in practice they result in being exposed to the heat produced by Joule effect on the circuit breaker itself. An excessive increase in the temperature of the circuit breaker can render it necessary to resort to the derating of the circuit breaker itself, i.e to an underuse compared to the nominal data, especially when it is installed inside a switchboard. Besides, it is nevertheless desirable to keep the operating temperature of the circuit breakers at low levels; it is known, in fact, that the higher is the operating temperature, the lower is the life span of the circuit breaker (or of its more sensitive components).
Normally, there is an attempt to solve such problems by increasing the dimensions and the volumes and by using materials that are particularly resistant to heat but are expensive.
Although these known solutions certainly provide some technical benefits, there is room and need for further improvements.